In pyrolytic cracking operations, feed stocks such as ethane, propane, naphtha, kerosene, gas oil, fuel oil and the like undergo “cracking,” that is the removal of hydrogen, to form unsaturated hydrocarbons. Pyrolytic cracking also tends to produce oxygenated hydrocarbons, including carbonyl compounds such as acetaldehyde. In a typical operation, the cracked effluent stream is quenched, fractionated and compressed. Acidic contaminants such as hydrogen sulfide, carbon dioxide and mercaptans are then typically removed from the effluent.
During the scrubbing operation of these gases with caustic or amine some oxygenated compounds are removed. At the same time, however, the basic conditions in the scrubber tend to cause base-induced condensation reactions (due to aldol condensation mechanism) of the carbonyl compounds, including aldehydes (e.g., acetaldehyde) and/or ketones, which in turn result in the formation of polymers. These polymers deposit on the internal surfaces of the scrubber. As the mass of polymer grows, it leads to fouling and can eventually obstruct the flow of liquids through the system. This is undesirable, as the operating system must be shut down for a significant amount of time in order to remove the deposited polymer and clean the equipment. This operation itself is very expensive involving many man hours and fmancial loss.
The prior art systems treat caustic towers with an injection of a compound to inhibit the aldol condensation mechanism. In order to inhibit aldol condensation the prior art systems mostly use additives that are organic in nature and contribute to chemical oxygen demand for caustic used in the scrubbing process.
In addition, the prior art additives typically require additive to reactant (i.e., carbonyl compound) molar ratios to be of at least about 1:1 for effective performance. Further the adducts of the high molecular weight polymers with these compounds tend to be insoluble in the basic system. Thus, the prior art additives are ineffective for the purpose of maintaining unobstructed flow through the system and reducing significant maintenance time for removing the polymer deposits and cleaning the equipment.
A current practice in the industry is to treat the weak caustic in the caustic tower with gasoline or another aromatic fraction in order to remove the polymers before sending it to the spent caustic oxidation unit, in order to prevent fouling there. The resulting gasoline-containing stream causes disposal and operational problem, however. Likewise, routing the gasoline-containing stream to other operating units can cause problems due to the presence of the caustic, as it may affect pH, catalyst and other plant parameters.
Another current practice in the industry is to treat the caustic tower with organic additives and despite the various advances in the art it remains desirable to provide an alternative method to improve the performance of this particular unit in the industry. There have also been shortcomings discussed later when any of these additives have been used concurrently to provide a synergistic effect in such systems.
In the past, prevention of polymerization of oxygenated compounds, such as carbonyl containing organics in basic solutions, has been attempted by process of inhibition only by adding amine compounds such as hydroxylamine hydrochloride, hydroxylamine sulfate, hydrazine, carbohydrazides and the like. Several patents which relate to methods of inhibiting carbonyl fouling due to polymerization are listed below. It is found that these patents discuss only the removal of carbonyl compounds but fail to address the issue of polymers which are formed as a result of polymerization of unscavenged portion of the carbonyl compounds during the use of inhibiting additive and also of polymers already existing in the system. Thus these patents discuss only the method of inhibition of polymerization.
U.S. Pat. No. 4,673,489 to Roling discloses using hydroxylamine and its salts of hydrochloric acid and sulfuric acid to inhibit polymer formation caused by condensation reactions of aldehydes contained in caustic scrubber units. One disadvantage of the method is that the additive has to be used in almost molar proportion. The other disadvantage is that these chemicals are expensive and must be over fed to the caustic wash unit system. This patent does not disclose a solution to the removal of carbonyl compounds and their polymers that remain unscavenged in the inhibition process, nor does it provide for removal of already existing polymers.
U.S. Pat. No. 4,952,301 to Awbrey discloses using ethylenediamines, with the molecular formula NH sub 2 (CH sub 2 CH sub 2 NH) sub x H were x is an integer ranging from about 1 to about 10, to inhibit carbonyl based fouling, particularly aldehyde fouling, that often occurs during caustic scrubbing of liquid or gas phase hydrocarbon streams in the base wash unit. This patent similarly does not provide a solution to removal of carbonyl compounds and polymers thereof that remain unscavenged during the inhibition process, nor does it solve the problem of removal of already existing polymers.
U.S. Pat. No. 5,264,114 granted to Dunbar also discloses the use of amine compounds to inhibit the deposition of foulants during caustic washing of the hydrocarbon gases contaminated with the carbonyl compounds. The method comprises of treating the said hydrocarbon gases with an aqueous amine solution, wherein 2 to about 5000 ppm of amine compound is selected from a group of organic compounds of the formula RNH2 and R2NH, R being selected from the group of alkyl or aryl groups. This patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.
Carbohydrazide has been disclosed as useful for inhibiting polymeric fouling deposits during the caustic scrubbing of pyrolytically-produced hydrocarbons contaminated with oxygen-containing compounds in U.S. Pat. No. 5,160,425 to Lewis. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.
U.S. Pat. No. 5,288,394 to Lewis and Rowe describes a method of inhibiting formation of polymeric fouling deposits after the caustic scrubbing of hydrocarbon stream contaminated with oxygenated compounds. The scrubbing is performed with a basic washing solution having pH more than 7, and comprising at least one hydrazide compound. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.
U.S. Pat. No. 5,194,143, granted to Roling describes and claims a method for inhibiting the formation of polymeric based fouling deposits during the basic washings of olefins containing hydrocarbon contaminated with oxygenated compounds comprising adding to the wash about 1 to 10000 ppm acetoacetate ester compound having the formula CH sub 3 COCH sub 2 c sub x H sub, where x is an integer from about 1 to about 8 and y is an integer from about 3 to about 17. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.
U.S. Pat. No. 5,220,104 to McDaniel at al. discloses the use of percarbonate salts for inhibition of fouling. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.
U.S. Pat. No. 5,770,041 to Lewis et al. describes the use of certain aldehydic compounds without alpha hydrogen atom or the use of non-enolizable aldehydes like formaldehyde, glyoxal and the like as aldol inhibitor. In this case the inhibitors are to be used at least thrice the molar ratio per mole of carbonyl species. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.
U.S. Pat No 5,710,455 to Bhatnagar et al. discloses the use of certain organic amine inhibitors like sulfanilic acid for inhibiting the aldol condensation. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.
All the patents of the prior art discussed above discuss treatment of the caustic with injection of a compound only to inhibit polymer formation by aldol condensation mechanism. They do not, however, solve the problem of removal of the polymers already present in the system.
Apart from the above mentioned disadvantages there are serious technical problems that exist with the prior art. For instance, one serious technical problem is the extremely rapid polymer formation; it typically takes place within few minutes, rendering impossible the complete scavenging of the carbonyl compounds by any known polymerization inhibition process. Obviously, an important requirement for inhibition of polymerization is that the inhibitor be present in the caustic tower before the carbonyl compounds enter the tower. Delay in supply of the inhibitor or incomplete availability of the inhibitor in the caustic tower will cause the carbonyl compounds to polymerize within few minutes, a process which is very detrimental to the unit leading to fouling. The unscavenged part of carbonyl compounds polymerizes and deposits on the trays, leading to fouling and plugging of the equipment, and eventually to equipment failure.
Another important technical problem is that the inhibitor used by other researchers can react only with nonpolymerized carbonyl compounds and with very low molecular weight caustic soluble species (2 or 3 repeating units of acetaldehyde), but not with high molecular weight polymers (having greater than 3 repeating units of acetaldehyde). It is precisely the high molecular weight polymers that are insoluble in the caustic system, thereby depositing and fouling the equipment. Hence there is a need to develop a method which that will not only inhibit the formation of polymers, but will also lead to dissolving the polymers already existing in the caustic tower and its downstream units.